1. Field of the Invention
The present invention relates generally to lasers, and more particularly, lasers emitting in the mid-infrared portion of the optical spectrum.
2. Description of the Related Art
Erbium lasers in the infrared near 2.9 microns may operate in either a Q-switched mode or in a free running (non-Q-switched) mode. The pulse duration at full width half maximum (FWHM) for erbium Q-switched laser pulses is generally in the range of 0.05-0.2 microseconds, which is short enough that a large acoustic effect may be generated. Such acoustic effects may be undesirable, depending on the target material. For example, acoustic effects in medical applications can lead to tearing of the affected tissue and/or adjacent tissues. On the other hand, the pulse duration at FWHM for free running, non Q-switched erbium lasers is generally in the range of 100-350 microseconds. Such a duration may be longer than optimal in some cases. For example, pulses longer than the thermal relaxation time (e.g., 10 microseconds) of certain human tissues can lead to unwanted thermal damage.
Although free running erbium lasers are quite effective at cutting materials having a high water content, such as tissue, one characteristic of such lasers impairs the cutting efficiency. As is well known, the output from a free running erbium laser is characteristically in the form of a series of intensity xe2x80x9cspikes,xe2x80x9d the maxima of which form an envelope that defines the laser output pulse. During tissue ablation, it is typical that the maxima of the spikes fall above, and the minima below, the ablation threshold of the tissue in question. Since ablation (cutting) occurs only above the ablation threshold, the energy below the threshold is wasted and results in undesirable heating of the tissue. Thus, there is need for an infrared laser which ablates tissue without causing either substantial collateral damage or acoustic damage to the tissue. To this end, it would be desirable to provide an erbium laser that emits laser pulses with a duration between that for Q-switched lasers and free running lasers.
According to one embodiment of the invention, a non-Q-switched laser that produces a series of laser output pulses includes a laser resonator, in which the resonator comprises a solid state optical gain medium for the wavelength range of 2.5 to 5.0 microns. The laser further includes an optical pump source that produces a plurality of pulses of optical radiation for pumping the gain medium, in which the pump pulses have an intensity and duration such that each of the pump pulses produces a respective laser output pulse comprising a dominant spike that contains at least a significant majority of the laser output pulse energy generated by its respective pump pulse, and preferably substantially all of such output energy. Each of the respective laser output pulses is separated from an adjacent laser output pulse by a time substantially equal to the time between pump pulses. In a preferred embodiment of the invention, the laser operates on the I11/2xe2x86x92I13/2 transition in erbium. In one preferred embodiment of the invention, substantially all of the laser output pulse energy is contained in the dominant spike. In a preferred embodiment of the invention, greater than approximately 90% of the energy of the laser output pulse is concentrated within the dominant spike.
According to another aspect of the invention, an erbium laser includes a laser resonator, with the resonator comprising a solid state optical gain medium, and the gain medium including erbium as a laser ion. The erbium laser further includes an optical pump source that generates a plurality of pulses of optical radiation for pumping the gain medium and producing laser output pulses, in which the pump pulses have an intensity and duration such that each of the laser output pulses has a full width half maximum (FWHM) of between 0.4 and 3 microseconds, and wherein each of the pulses is separated from an adjacent pulse by a time substantially equal to the time between pump pulses. In a preferred embodiment of the invention, the gain medium comprises a solid state host selected from the group consisting of YAG, YLF, YSGG, and YAP, and the laser output pulses from the laser have a wavelength near 2.9 microns. In a preferred embodiment, the optical pump source comprises a flashlamp, in which a voltage is applied to the flashlamp for a given duration, with the voltage and the duration thereof controlling the temporal behavior of the laser output pulses.
Another aspect of the invention comprises a method of generating laser pulses, in which the method includes providing a laser resonator that includes a solid state optical gain medium for the wavelength range of 1.9 to 5.0 microns, pumping the gain medium with pulses of optical radiation to optically excite ions in the gain medium, and generating optical feedback to create laser output pulses, in which the intensity and duration of the optical pump pulses is such that the laser output pulses have a FWHM of between 0.4 and 3 microseconds, and wherein each of the pulses is separated from an adjacent pulse by a time substantially equal to the time between pump pulses.
According to yet another aspect of the invention, a laser includes a non-Q-switched laser resonator, with the resonator comprising a solid state optical gain medium for the wavelength range of 1.9 to 5.0 microns. The laser further comprises an optical pump source that generates a plurality of pulses of optical radiation for pumping the gain medium and producing laser output pulses, in which the pump pulses have an intensity and duration such that each of the laser output pulses has a FWHM of between 0.1 and 10 microseconds, and wherein each of the pulses is separated from an adjacent pulse by a time substantially equal to the time between pump pulses. In a preferred embodiment of the invention, the FWHM of the laser output pulses is between 0.2 and 0.5 microseconds. In another preferred embodiment, the FWHM of the laser output pulses is between 0.1 and 0.3 microseconds.
According to still another aspect of the invention, an erbium laser includes a laser resonator, in which the resonator comprises a solid state optical gain medium for providing optical feedback, with the gain medium including erbium as a laser ion. The erbium laser further includes an optical pump source that produces a plurality of pulses of optical radiation for pumping the gain medium, in which the pump pulses have an intensity and duration such that the erbium laser produces laser output pulses having fewer than 10 transverse modes. In one preferred embodiment of the invention, the laser output pulses have fewer than 5 transverse modes.
Another aspect of the invention comprises a method of generating erbium laser pulses that includes providing a laser resonator having an optical gain medium for the wavelength range of 1.9 to 5.0 microns, pumping the gain medium with pulses of optical radiation to optically excite erbium ions in the gain medium, and generating optical feedback to create laser output pulses, in which the intensity and duration of the optical pump pulses is such that each of the laser output pulses comprises several dominant spikes having intensities of the same order of magnitude, wherein the number of spikes is no greater than 5 and the pulses have a FWHM of between 0.4 and 20 microseconds, and wherein each of the pulses is separated from an adjacent pulse by a time substantially equal to the time between pump pulses.
According to yet another aspect of the invention, a non-Q-switched laser that produces a series of laser output pulses includes a laser resonator, in which the resonator comprises an optical gain medium that includes erbium as a laser ion in a solid state host. The laser further includes an optical pump source that produces a plurality of pulses of optical radiation for pumping the gain medium. The pump pulses have an intensity and duration such that each of the pump pulses produces a respective laser output pulse having rise and fall times of 1 microsecond or less, and wherein each of the respective laser output pulses is separated from an adjacent laser output pulse by a time substantially equal to the time between pump pulses. The laser output pulses may advantageously have a wavelength on the order of 3 microns.
Another aspect of the invention comprises a method of corneal sculpting that includes directing a beam of infrared laser light having a wavelength of between 1.9 and 5.0 microns onto corneal tissue. The beam is supplied in the form of pulses, each of which (a) has a peak energy density above the ablation threshold for the corneal tissue but whose energy density is sufficiently low to limit the ablation rate to no more than about 3 microns per pulse, (b) has a pulse duration whose full width half maximum (FWHM) is greater than about 300 nsec and is less than the thermal relaxation time of untreated corneal tissue, and (c) has a rise time and a fall time which are both less than 10 microseconds. The method further comprises ablating a surface of the corneal tissue using the laser light, whereby no substantial tissue damage occurs more than 30 microns below the ablated surface.